The present invention relates to a telephone capable of limiting a band-width of a received speech signal and, more particularly, to a mobile telephone capable of making a received speech easy to hear even in a noisy environment.
It has been customary with a telephone to include a filter for limiting a band-width of a received speech signal in a receiver circuit thereof. The filter limits a received speech signal to a predetermined frequency band and thereby makes the speech easy to hear. Specifically, the telephone has a casing and a transmitter (microphone) and a receiver (speaker) mounted on the casing. A radio section demodulates a signal coming in through an antenna. The filter mentioned above controls the frequency band of the output of the radio section. The radio section and filter are connected to the receiver. In this configuration, the demodulated signal from the radio section has the frequency band thereof limited by the filter, so that a speech signal is output from the receiver within the limited frequency band. Usually, the filter is of the kind filtering out a low frequency component in order to make a received speech easy to hear.
Various approaches have recently been proposed to improve the quality of speeches received on a telephone. One of them is to provide the filter with a characteristic which enhances a low frequency component by filtering out a high frequency component. Specifically, when a high frequency component is enhanced to promote easy hearing, a received speech sounds metallic and annoys the user of the telephone. In contrast, the filter of the kind filtering out a high frequency component allows a received speech to sound soft, thereby improving the speech quality. Another approach is to allow the user to adjust the frequency of a received speech on the telephone, i.e., the easiness of hearing, as taught in Japanese Patent Laid-Open Publication No. 1-123554 (document 1) by way of example.
However, the telephone with any of the conventional implementations for improving speech quality has the following problem. Assume that the telephone is used in a noisy urban environment outside of a building. Then, because city noises are generally distributed in a low frequency component, they are superposed on a speech signal whose frequency is limited to a low frequency component. The resulting speech output from the receiver is extremely hard to hear. For this reason, it has been customary to limit received speeches to a high frequency band so as to promote easy hearing, rather than to improve the speech quality in consideration of operation in noisy environments.
The telephone taught in the above document 1 promotes easy hearing because it allows the frequency characteristic of a received speech to be adjusted in matching relation to the environment. However, the adjustment is done by hand and, therefore, forces the user to manipulate the telephone while conversing with the other party. Moreover, it is likely that noise drowns out a speech due to inadequate adjustment and shuts off communication for a moment.